Can't Copy Paste Mirrored Pose
In order to perform any kind of mirroring operation on your bones the names on the left and right side of your rig must have identical names, other than the L/R indication. All the bones on the right side of your armature have '.001' at the end of the names left over from being copied at some point. Remove the .001 from each of the names on the right side so they match the left side and you will be able to paste the pose mirrored.